1. Field of the Invention
The present invention relates to a steel wire material for a spring wherein ferrite decarburized layer is not substantially present and workability is excellent, and its producing method.
The present invention further relates to steel for spring (spring steel) useful as a material for a coil spring used in a heat treated (quenched and tempered) condition, and more specifically, to a steel wire material for a spring excellent in corrosion fatigue property.
2. Description of the Related Art
In the steel wire material for a spring which requires high fatigue strength, high alloying is generally directed, and in addition, much Si is added to improve yield strength ratio of an element wire for the spring after quenching and tempering. However, because addition of a large amount of Si narrows the austenitic zone in the phase equilibrium diagram, ferrite decarburization is liable to occur.
To inhibit ferrite decarburization with austenitic zone being widened, alloy elements such as Ni, Cu, Mn may be added. However, only adding of these alloy elements enhances hardenability of the wire material too much and the metastable structure such as bainite and martensite is liable to be generated in the cooling process after hot rolling. This metastable structure exerts bad influence upon wire drawing (especially upon large diameter wire material) and causes cuppy break or transverse crack fracture.
In this connection, a variety of technologies have been proposed for preventing ferrite decarburization while maintaining excellent workability. For example, the Japanese Unexamined Patent Application Publication (JP-A) No. 2002-194432 discloses a technology for preventing ferrite decarburization by maintaining the steel temperature in the temperature range higher than the A3 transformation point in all steps from the beginning to the end of hot rolling and the cooling speed after hot rolling is set at 0.5° C./s or faster, is disclosed. Further, the patent documents discloses the cooling speed should be 3.0° C./s or slower to lower the hardness of the wire material and to improve workability.
Also, in the JP-A-2007-009300, technology for preventing ferrite decarburization by performing rapid cooling in the temperature range of decarburization region between the A3 transformation point and the A1 transformation point (eutectoid transformation point) in the cooling process of the wire coil is disclosed. The patent document further discloses a technology for enhancing workability of the wire material at ordinary temperature by promoting pearlite transformation by performing slow cooling after the rapid cooling.
For the coil springs used for an automobile and the like, weight reduction is required for exhaust gas reduction and fuel economy improvement, and increase in the strength is directed as a part of it. In the spring with increased strength (the spring with the tensile strength after quenching and tempering is, for example, 1,900 MPa or more), early breakage by hydrogen embrittlement and corrosion fatigue generally becomes a problem.
To solve such problems, a variety of technologies have been conventionally proposed. For example, although Cr is generally known as an element for enhancing anti-corrosion property, the JP-A-2002-047539 discloses that the tensile test under a low distortion speed after a saline water spraying cycle test shows that addition of Cr inversely may reduce anti-corrosion property and Cu and Ni are effective to enhance anti-corrosion property in such case, and proposes to make the total amount of Cu and Ni to be two times or more of Cr.
The JP-A-2004-010965 teaches that C is to be decreased with the reason that C causes lowering of corrosion fatigue strength, deterioration of settling resistance which is worried due to decrease of C is to be prevented by adding Si, Cu, Ni, and etc., and Cu and Ni are effective in enhancing anti-corrosion property as well.
However the technical level of the knowledge described in these two patent documents is not high enough and there is room for further improvement in corrosion fatigue strength. For example, according to those patent documents, Ni is recognized to simply be excellent in anti-corrosion property and detailed study on its detailed interactive mechanism as well as on merits and demerits are lacking. Elements other than Ni can be considered to be the same.
Although a variety of conventional technologies have been proposed as described above to prevent ferrite decarburization, those effects are insufficient. For example, in the EXAMPLE column of the JP-A-2002-194432, the ferrite decarburization depth is shown to have attained 0 mm, but Si content of the steel used then is 1.79 wt % which is comparatively little. Also, in the JP-A-2007-009300, the ferrite decarburization depth is shown to have attained 0 mm, but C content of the steel used then is 0.48 wt % which is comparatively much. When C content is much or Si content is little, because the ferrite band in the Continuous Cooling Transformation (CCT) curve becomes thin, ferrite decarburization is comparatively easy. As the applicable element series of the technologies described in the JP-A-2002-194432 and No. 2007-009300 are limited, further development in the preventing technology of ferrite decarburization is desirable.